Data from: Body size evolution in an old insect order: no evidence for Cope’s Rule in spite of fitness benefits of large size

We integrate field data and phylogenetic comparative analyses to investigate causes of body size evolution and stasis in an old insect order: odonates (“dragonflies and damselflies”). Fossil evidence for “Cope's Rule” in odonates is weak or non-existent since the last major extinction event 65 million years ago, yet selection studies show consistent positive selection for increased body size among adults. In particular, we find that large males in natural populations of the banded demoiselle (Calopteryx splendens) over several generations have consistent fitness benefits both in terms of survival and mating success. Additionally, there was no evidence for stabilizing or conflicting selection between fitness components within the adult life-stage. This lack of stabilizing selection during the adult life-stage was independently supported by a literature survey on different male and female fitness components from several odonate species. We did detect several significant body size shifts among extant taxa using comparative methods and a large new molecular phylogeny for odonates . We suggest that the lack of Cope's rule in odonates results from conflicting selection between fitness advantages of large adult size and costs of long larval development. We also discuss competing explanations for body size stasis in this insect group.本研究整合野外实测数据与系统发育比较分析方法，探究古老昆虫类群——蜻蛉目（odonates，即蜻蜓与豆娘）的体型演化与停滞现象。自6500万年前末次大灭绝事件以来，支持柯普法则（Cope's Rule）的蜻蛉目化石证据仍十分薄弱甚至完全缺失，但现有选择研究均表明，成虫体型增大受到持续的正向选择作用。具体而言，本研究通过多代观测发现，斑翅丽蟌（Calopteryx splendens）自然种群中的大型雄性个体，在存活率与交配成功率两方面均具有稳定的适合度优势。此外，成虫阶段内各适合度组分之间未表现出稳定选择或拮抗选择的相关证据。针对多个蜻蛉目物种雌雄个体不同适合度组分的文献调研结果，独立验证了成虫阶段缺乏稳定选择这一结论。本研究通过比较分析方法与全新的大规模蜻蛉目分子系统发育树，在现生类群中检测到了多起显著的体型演化偏移事件。我们认为，蜻蛉目类群缺乏柯普法则支持的原因，在于大型成虫所具备的适合度优势与幼虫长期发育所付出的代价之间存在拮抗选择。本研究同时探讨了该昆虫类群体型停滞现象的其他竞争性解释假说。